1. Field of the Invention
The present invention relates to a vibration wave drivenmotor. More particularly, the present invention pertains to the arrangement of an electrode pattern for an electro-mechanical energy conversion element, such as a piezo-electric element, bonded to an elastic body.
2. Related Background Art
The basic structure of a vibration wave driven motor which utilizes a travelling vibration wave will be described schematically with reference to FIGS. 7 and 8.
In FIGS. 7 and 8, a reference numeral 1 denotes an elliptical elastic body which serves as a vibration body and which consists of straight portions and curved portions. The elastic body is made of an elastic material, such as a metal. In the case where the vibration wave driven motor is employed as, for example, a sheet conveyance means for a printer, it will be arranged such that a sheet can be conveyed linearly by the straight portion. A reference numeral 2 designates an electro-mechanical energy conversion element, such as a PZT, which consists of straight portions and curved portions. Hereinafter, the electro-mechanical energy conversion element is typified by a piezo-electric element. An electrode film 3, made of Cu, Ni or Ag, is bonded to the two surfaces of the piezo-electric element 2 by means of printing or deposition. The electrode film 3 bonded to the rear surface of the piezo-electric element (which is not bonded to the elastic body) has a pattern having cuts 3A. The electrode film 3 formed on the front surface of the piezo-electric element 3 (which is bonded to the elastic body) has no cuts and thus forms an overall surface electrode.
The electrode pattern 3 formed on the rear surface of the piezo-electric element 2 will be further described below with reference to FIG. 8. The example illustrated in FIG. 8 produces a travelling wave which is a combination of two sets of standing waves each of which is characterized by the presence of six points of successive maxima and minima in the peripheral direction. The electrode pattern 3 shown in FIG. 8 includes a first driving electrode group consisting of electrodes 3a1 and 3a2, a second driving electrode group consisting of electrodes 3b1 and 3b2, a grounding electrode 3G and sensor electrodes 3Sa and 3Sb which respectively detect vibrations generated by the first and second electrode groups. Each electrode in both the first and second electrode groups has a length equal to one half of the wavelength and is disposed at a pitch equal to one half of the wavelength. The first electrode group is shifted from the second electrode group by a distance which is an odd multiple of one quarter of the wavelength. The cuts are formed in a radial fashion in each of the curved portions. The portions of the piezo-electric element 2, which correspond to the respective driving electrodes 3a1, 3a2, 3b1 and 3b2 in the first and second electrode groups, develop polarization in the direction of its thickness. At that time, the portions of the piezo-electric element 2, corresponding to the electrodes 3a1 and 3a2 in the first electrode group, develop polarization in opposite directions. Similarly, the portions of the piezo-electric element 2, corresponding to the electrodes 3b1 and 3b2 in the second electrode group, develop polarization in opposite directions. That is, when a voltage having the same polarity is applied to the first electrode group, the portions of the piezo-electric element 2 corresponding to the electrodes 3a1 and 3a2 develop polarization such that they are expanded or contracted in opposite directions. The same thing applies to the portions of the piezo-electric element 2 corresponding to the electrodes 3b1 and 3b2.
Thus, supply of voltages which are out of phase by 90.degree. to the electrode groups generates in the elastic body 1 a travelling wave which is a combination of two sets of standing waves and which frictionally drives a moving body (not shown), such as a sheet or a rotor, pressed to the elastic body 1. If the moving body is made fixed, the elastic body 1 is made movable.
The above elliptical elastic body has two vibration modes. FIGS. 9 and 10 are respectively contour maps of deviation generated in the two vibration modes, obtained by the characteristic value analysis in the finite element method. The vibration mode shown in FIG. 9 is obtained by driving the first electrode group and the vibration mode shown in FIG. 10 is provided by driving the second electrode group. Deviation represents a component in a direction perpendicular to the surface of the piezo-electric element 2. The contour lines shown in FIGS. 9 and 10 are those of the surface of the piezo-electric element. Deviation is normalized with the maximum value as "1."
As is clear from the contour maps of FIGS. 9 and 10 and the electrode pattern shown in FIG. 8, the nodes of the vibrations (indicated by thick lines in FIGS. 9 and 10) shift from the cuts in the electrode pattern (see FIG. 8). As a result, the force which generates deviation also acts on the nodes, thus reducing efficiency of the motor.